This invention relates generally to wireless communication systems and more particularly, to wireless voice communication systems.
In wireless communication systems, a portable or mobile communication unit (e.g., portable or mobile radio or telephone) may be configured to allow a member of a group of related units to transmit a voice message to a base station, with the base station then broadcasting the message to the other members of the group. For example, when one person talks on their unit, the other group members receive the communication and can listen on their units. Alternatively, a portable or mobile unit may communicate with only one other unit.
A mobile communication unit (e.g., mobile telephone), in contrast to a portable communication unit, typically receives power from the battery of a vehicle. A portable communication unit typically receives power from the battery of the portable unit. In operation, when a communication unit operates in half-duplex mode, the unit transmits or receives voice, but does not transmit and receive voice simultaneously. Thus, in half-duplex mode, a user desiring to talk, pushes, for example, a push-to-talk button on the unit allowing the user to talk, while one or more users receive the communication and those users are only able to listen to the communication. Alternatively, when operating in a full duplex mode, voice is transmitted and received by the unit simultaneously. When operating in full duplex mode, for example, the unit communicates with another unit operating in full duplex mode allowing simultaneous transmission and reception of voice.
The quality of wirelessly transmitted voice may be affected by multi-path fading, which produces errors in the digital transmission of digitized compressed voice packets (also known as voice blocks). The multi-path fading duration may be relatively short (<5 milliseconds (msec)) for a transmission from a mobile phone unit traveling at relatively fast speeds (e.g., 60 miles per hour (mph)) as compared to the multi-path fading duration for a transmission from a portable phone unit moving at relatively slow speeds (e.g., 5 mph). Thus, depending on whether the user of the communication unit is, for example, travelling in a car or walking, the duration of possible multi-path fading is affected. It is known to use error detection and correction codes, such as, for example, a Reed Solomon code, to detect and correct errors within a voice packet. However, these error detection and correction codes are not always effective in correcting errors in voice packets having long duration multi-path fades. Further, the effectiveness of the error detection and correction is also affected by the type of communication unit. For example, a portable phone unit usually transmits with much less power than a mobile phone unit because a portable phone unit typically uses its own internal battery power, thus, increasing the problem with multi-path fading.
It is also known in packet technology to use retransmission of packets to improve transmission quality when packet errors are detected. In this scheme, and when error detection and correction codes are unable to correct for errors in transmission, a request is made to the transmitter to retransmit, for example, the packet that failed to be transmitted and received in a useable form. A form that is not useable may be a packet received with errors that can not be corrected by an error detection and correction scheme. Herein, a packet received in a useable form is equivalent to a packet received in a correctable form. However, with respect to voice packets, voice latency for retransmission of voice packets causes delays, thereby causing a noticeable delay to a user receiving the voice packets (e.g., delay in received voice). For example, when real-time voice is transmitted (as compared to transmitting streaming voice from a storage medium), the transmission needs to be received by the human user typically within a quarter of a second from generation at the source in order for voice delay to be unnoticeable. Retransmission of voice packets for packets that failed to be transmitted and received in a useable form may cause voice delay in the conversation to be greater than a quarter of a second, which becomes noticeable and annoying to the human users. In real-time packetized voice communications, the received voice transmission is not provided as a continuous stream, and increased voice latency (e.g., due to packet delays), may reduce user satisfaction and may be unacceptable to certain users (e.g., time sensitive conversations).